GRAIL: The Search for the Holy Moonsplat

On New Year’s Eve, Maria Zuber and her team of 60 were not partying with friends, family and a glass of bubbly. They were at work, at Mission Control at the Jet Propulsion Laboratory (JPL) in Pasadena, California, but no one complained. They were putting a pair of washing-machine-sized probes into orbit around the Moon, and the operation went perfectly. Launched on September 10th, GRAIL-A was inserted into a near-polar, highly elliptical lunar orbit on New Year’s Eve, and GRAIL-B followed on the afternoon of New Year’s Day. Finally, it was time to celebrate.

GRAIL, which stands for “Gravity Recovery and Interior Laboratory”, is a pair of spacecraft which will measure the Moon’s gravity in unprecedented detail over a three-month period, effectively X-raying the moon from crust to core. In a sense, “GRAIL is a Journey to the Center of the Moon,” explained Zuber, a geophysics professor at MIT. After the spacecrafts’ orbits are made nearly circular, they will fly in formation 34 miles above the lunar surface, precisely measuring the range between the two. The spacecraft will use variations in their own orbits to map density changes within the Moon. Producing data with quality between 100 and 1000 times better than what we already have, they will even be able to look for a slight periodic deformation in the shape of the Moon when it reaches its closest approach to the Earth, caused by the squeeze of Earth’s gravity. These measurements could indicate whether the Moon’s interior is still partially molten or has completely solidified.

Complementing the high-resolution imagery of the Moon’s surface taken by the Lunar Reconnaissance Orbiter, GRAIL’s science objectives are to determine the structure of the Moon’s interior and to advance our understanding of the Moon’s thermal evolution. The mission is motivated by a panoply of lunar mysteries. Like the question of whether there’s still a layer of magma within the Moon, it’s amazing how much we don’t know about our nearest celestial neighbor.

The lunar near-side is dominated by smooth, relatively uncratered dark maria with some major highland areas. (Photo by author, 1/6/2012. Click for the very large full-sized image.)

The biggest mystery of all is why the far side of the Moon is so very different from the near-side. Tidally locked to the Earth, the Moon rotates exactly once per orbit, always presenting the same familiar face to us. Low, dark maria cover much of the Moon’s near-side, surrounded by the paler, heavily cratered and rayed lunar highlands. But the far side is all mountainous highlands, with hardly any maria and a crust that seems to be much thicker than the near side. What could cause such a dramatic difference? Since the Soviet Luna and U.S. Apollo missions saw the Moon’s surprisingly lopsided rear for the first time, scientists have scrambled for an explanation.

GRAIL seeks to map out the Moon’s crust in enough detail to put the mystery to rest. A model developed by Martin Jutzi and Erik Asphaug of UC Santa Cruz suggests that the thick crust and mountains on the lunar farside are actually the remnants of a collision, early in the Solar System’s history, with a second smaller moon of the Earth. According to the hypothesis, both moons formed at the same time, sharing the same orbit. When the smaller moon eventually collided with its larger sister, the collision was low-speed. Rather than vaporizing on impact and creating a huge crater, the smaller moon splattered into a layer of rock which forms the thick crust seen on the present-day Moon. This impact could have happened on any side of the Moon, but much later, when the Moon became tidally locked to the Earth, the side with the thicker crust was oriented to point outward.

Snapshots of Jutzi & Asphaug's computer simulations show how the smaller moon could be pancaked onto the Moon by a low-speed collision. (Credit: M. Jutzi & E. Asphaug, Nature.)

Could the Moon’s mysterious back face actually be the body of a long-gone second moon of the Earth? GRAIL is searching out the answer. The Moon is not the only two-faced moon of the Solar System, though it is a very different process that causes a dramatic color contrast on Saturn’s moon Iapetus. Ironically, though several moons have differently-colored hemispheres for various reasons, Saturn’s small moon Janus, named after the two-faced god himself, is not one of them.